Acorn Archimedes

The Acorn Archimedes is, or rather was, a range of computers produced
from the late 1980s to mid 1990s by Acorn Computers ltd. of
Cambridge, England. The successor to the original 6502-based BBC
Micro, the Archimedes machines were designed around the
original production 32-bit ARM2 processor and its chipset.

Initially released in August of 1987, it was the world's first microcomputer
with a RISC processor; and this was at a time when RISC was still
considered a valid design philosophy rather than a worn-out
buzzword.

Evolution of the Archimedes

The Archimedes specification and design evolved from various
commercial failures, the Cambridge
Workstation version of the unsuccessful Acorn
Business Computer in particular. Staff at a short-lived Acorn research facility
in Palo Alto were, inspired by work at Xerox, working on an
operating system known as ARX, which was to be the operating
system for the successor to the Acorn Business Computer. As can
probably be guessed from the 'research' label, no working operating
system ever emerged. Looking to cut their potential losses, Acorn
approached the BBC with their designs, and a plan for a stopgap
operating system (Arthur), offering the system as the
true replacement for the BBC Micro.

"The BBC said it was interested, so we put some red function keys on
it and changed the case colour. Our original research for the office
automation machine had said it had to be grey."

-- Roger Wilson

Production Systems

The BBC liked the system, and so with a market assured, Acorn put the
new machines into production. The initial product line consisted of the
A305 and A310, essentially the same systems but with 512k and 1Mb of
RAM respectively. The A410 and A440 were similar in design but
included an on-board ST506 hard disc controller. The A410 had 1Mb of
memory on board, the A440 had 4Mb, and a 20Mb hard disc drive.
These were later revised, by the addition of larger hard discs and a
faster version of the memory controller chip MEMC1a, to become
A410/1 and A440/1. At the same time, the A305 and A310 were dropped,
and the low-cost A3000 introduced (which, like the BBC Micro, and
its contemporaries the Commodore Amiga and Atari ST, had the
mainboard, disk drive and keyboard integrated into a single unit]).

The Archimedes Chipset

The Archimedes chipset used a unified 8MHz bus architecture, with a single
main bank of RAM containing all program and data memory, as well as
video and sound buffers. DMA to video and sound was handled by the
VIDC (VIDeo Controller) chip, in conjuction with the MEMC
(MEMory Controller, also known as 'Anna') chip. The MEMC provided basic DMA facilities,
memory timing and refresh, controls, as well as an inverse page
table memory mapping system, allowing sophisticated memory
management of up to 4Mb of fast page modeDRAMs. This 4Mb limit
quickly became painful to Archimedes owners. Many machines could handle more than 4Mb, but only with the addition of a separate MEMC chip for each additional 4Mb of RAM. The VIDC (also known,
for reasons probably lost in the annals of time, as 'Arabella')
could produce 12-bit colour outputs, and had a set of palette registers
which provided 16 palette entries to 4-bit (16-colour) video modes, or
provided part of the palette data for 8-bit/256-colour video
modes. The fixed part of the palette data for 256-colour modes
gave a very well-rounded display, and with a little colour dithering
(built into RISC OS) was almost indistinguishable from most 16-bit
displays.

The chipset was rounded out by the IOC ('Albion'), or Input/Output Controller,
which provided miscellaneous input/output functions, timers and timer
comparison interrupts.

The Last Archimedes

In 1990, a new flagship machine took over from the A440/1, namely the
A540. The main new feature of this machine was the use of the new
ARM3 processor instead of the ARM2 used in previous machines: this
ran at 25 or 33MHz, and featured a unified 4Kb level 2
instruction/data cache; theoretically, it made the machine up to six
times faster than the 8MHz ARM2s. This magnitude of speedup was
frequently realised in practice, helped by the density of ARM
instructions, and by reducing the CPU's dependency on main memory bandwidth also required for tasks such
as video DMA. The A540 also featured (naturally) a larger hard disc,
and an option to install an additional MEMC memory controller chip, giving
access to up to 8Mb of RAM.

1991 saw the release of the A5000, which was the first machine to
make use of a revised chipset, which replaced the MEMC and IOC chips
with an IOMD (Input/Output and Memory Device) chip, and also came
with a much revised version of RISC OS.

With the A5000, the 'Archimedes' moniker was officially dropped, making the A540 the last true 'Archimedes' machine; but
since the product line continues, so shall I...

Hot on the heels of this machine was the A4, Acorn's only RISC OS
laptop to see the light of day. Its design was based on an Olivetti
laptop chassis with a 640x480 grayscale LCD monitor, and the same
chipset as the A5000. A commercial failure due to its high price and
lack of flexibility; although this could be said of almost any Acorn
machine since the BBC Micro, it was particularly true of the
conspicuously overpriced A4.

Mass Market Failures

The next machines, announced in late summer 1992, were aimed at
reducing costs, both retail and manufacture. The basic Archimedes
chipset had fallen behind the times. Silicon processes were shrinking
while Acorn used essentially the same chips, without dramatically
improving performance. The shrinking process sizes, however, gave an
opportunity for a significant cost saving without extra work to
further improve performance. The entire chipset, ARM2, VIDC, MEMC1a and
IOC were integrated onto a single chip, vastly reducing the
complexity of system boards and the chip count. Also, since memory
technology had improved in the interim, the system bus and system
chip run at 12MHz instead of the previous 8MHz). This all-in-one system
chip, the ARM250, was used in the A3010, A3020 (both models
billed as replacements for the A3000, differentiated by the
expandability of RAM, inclusion of an on-boardIDE controller only
on the A3020, and the fact that the A3010 had green function keys
rather than the traditional red.) and the A4000 which was in
appearance much like a slim-line A5000.

Then things got quiet for a good long time, until the RiscPC was
unveiled in 1994. The RiscPC was a breakthrough in its day, and
brought workstation class features to the home computer market:
massively expandable memory capabilities, a multi-processor bus
capable of accepting both multiple ARM processors or an Intel486
as a second processor to run Windows software. Processor cards
included the new ARM6 and ARM7, and plans were in place for
ARM8, although these were largely scuppered by the introduction of
cards for Digital Semiconductors' (now Intel) StrongARM
processor. The RiscPC featured a much improved video controller chip, VIDC20
which could support far higher resolution and colour depths than the
VIDC10's 256 colour limitation. For the first time on an Acorn
RISC OS machine, the video subsystem had its own private memory and
data bus, which allowed higher bandwidth displays to be used without
impacting the processor's performance unduly.

The weak point of the RiscPC's design, however, was its cripplingly
slow CPU and system bus. The bus was still 32 bits wide, and ran at
16MHz. In comparison with the baseline Pentium system's bus of 64 bits
and 33MHz, this just wasn't sufficient for a scalable system, and in
particular was not sufficient to adequately support the 2-way
multiprocessing which the bus architecture was capable of physically
supporting.

The last machine to make it out of Acorn's workstation division
before they closed completely was the A7000. This was similar in many
ways to the A4000, being a small-footprint desktop case with an
integrated system chip, the ARM7500 (or ARM7500FE with floating
point accelerator). Several models were released, some intended as
diskless network nodes, all models having ethernet interfaces built in.

Shortly before the closure of the workstation division, the
development of the RiscPC's successor, named 'Phoebe' (Yes, after the
character from Friends...) was almost completed. This machine
featured standard PC components where custom ones had previously been
required, and included such joys as PCI slots, proper 16-bit sound,
and a yellow system case. None were ever sold.

And there ends the story of one of the greatest, most innovative lines
of home computers ever built, and never sold.

Despite being relatively small compared to their industry peers, Acorn Computers were an innovative team. From their headquarters in Cambridge, they had already released the Atom, before following up in 1982 with their hugely successful Proton, otherwise known as the BBC Microcomputer. They had also branched out into new markets with the ABC, or Acorn Business Computer.

The ABC was not an astounding success, and certainly didn't meet the company's expectations. Nor did the Acorn Electron - a stripped-down BBC Micro intended for home use. While demand for the Electron was high, difficulties in producing the custom microchips used in its design meant few were sold - and, once stocks had risen to satisfactory levels, demand had long peaked.

One of the great strengths of the BBC Micro was the Tube, another Acorn development. The Tube was a fast bus that allowed for many different second processors to be installed, such as a Z80 or 80186. This allowed the BBC Micro to run different OSes, such as CP/M. Much of Acorn's development at this stage was focused on running more and better hardware off the Tube, but Acorn had seen the need for a powerful new solution if they were to succeed in the market from as early as the launch of the Atom in 1980.

The venerable 6502, upon which the Proton was based, was by 1983 quite long in the tooth. Acorn wanted a powerful, flexible new computer system, complete with a graphical user interface such as those Xerox had been experimenting with. Having asked Intel for 80186s to use, they had been turned away - and the 68000 was underwhelming.

The breakthrough came when Acorn engineers read the Berkeley RISC documents. The decision was swiftly made: if off-the-shelf processors don't make the grade, roll your own. Sophie Wilson, Acorn's virtuoso engineer, set to work, writing the processor's instruction set on a twin-6502 Beeb.

After the company was bought by Olivetti, full-scale efforts towards producing the new RISC processor - dubbed the Acorn RISC Machine, or ARM - were launched. VLSI Technology, a firm Acorn already contracted to produce various chips, such as ROMs, were asked to produce an ARM. On 26 April, 1985, the processor was delivered to Acorn - and, perhaps an omen of future success, it worked first time. The ARM1 was ready.

Using the ARM1 via the Tube as a co-processor in BBC Micros, the ARM2 was swiftly designed. The ARM2 was a 32-bit microprocessor, with 32-bit bus and 26-bit address space. The ARM2 was also Tube-compatible, and so it was decided that it should be implemented in development machines for the new Acorn computer: the Archimedes. The ARM2 was a top-secret project still at Acorn: even when courting Olivetti, they hadn't let on that it was under development.

The ARM Development System was released in 1986, at £4,000. A Tube-compatible co-processor for the BBC Master, it boasted an ARM2 which ran rings around the 286. It also came with three support chips, and 4MB of RAM. Acorn continued development of the Archimedes; now their wünderkind was ready, in the shape of the ARM, they needed the OS to run on it. This task was given to the Acorn Research Centre, or ARC, at Palo Alto, California.

Work on the OS, known as ARX, was slow. Acorn's goal of a UNIX-like OS would have had a truly modern platform, capable of pre-emptive multi-tasking, multi-threading, and multiple users. However, by 1987 it was clear that ARX would not be ready in time. A stop-gap was sought.

Hurridly, using the legendary BBC Basic which Sophie Wilson had ported to the ARM (which led to truly fast execution times), an OS was written, complete with GUI. With a gaudy, bright colour scheme reminiscent of early CommodoreAmiga Workbench screens or Windows 2, it nonetheless got the job done. It featured an icon bar, displaying devices and applications, and windows for each currently-running application. Its name: ARTHUR, commonly believed to stand for "ARISC OS before THURsday".

Now Acorn had their hardware and software ready. In June, 1987, their ARM-based computer was launched: the Acorn Archimedes. The Archimedes, or Archie as they were affectionately known as (or, more often, simply Arc), came in four models to begin with, all equipped with an ARM2. The A305, with only 512k RAM and two expansion slots, was complimented by the A310 with a full megabyte. At the upper end, the A410 boasted 1MB RAM but four slots, whilst the top-of-the-line A420 boasted not only four slots and 2MB RAM, but a whopping 20MB hard disk. All four shipped with ARTHUR in ROM. Later, the A440 and A540 were released, with 4MB RAM and a 40MB hard drive, and 4MB and a 100MB hard drive, respectively.

The Archimedes was a desktop system, with the actual computer designed to sit on a desk, the monitor atop that. A keyboard plugged into this, and into that a three-buttoned mouse. The OS, to allow it to run on all systems (and not just those with hard drives), was burned onto ROMs. This allowed for swift boot times and operation, but was more difficult to upgrade. Nevertheless, Acorn began work on a proper, much less buggy OS for the Archimedes. The result of this was the RISC OS.

Originally to be named Arthur 2, this idea was scrapped when a film with a similar name was released. RISC OS 2 saw release in 1989, available on a set of ROMs to replace the original ARTHUR. RISC OS is a fast and easy-to-use OS, which featured many ideas ahead of their time. All applications are provided as folders, which carry all their resources. Double-clicking one in the Filer will run the app, whilst holding Shift when double-clicking will open the app's folder. All fonts are antialiased - a miracle in 1989. A command line could be accessed at any time just by tapping F12.

To go with the new OS, new hardware was designed, leading to the launch of the A3000. Still sporting the BBC branding, the Acorn Archimedes A3000 was an all-in-one unit like the Atari ST - floppy drive, computer and keyboard in one unit. The A3000 was an astounding success, with a powerful 8Mhz ARM2 and 1MB RAM, and sold well to both home and educational markets. The A3xx series was phased out.

ARM development hadn't stalled while Acorn worked on RISC OS - instead, they had produced a third iteration of the hardware. The ARM3 was a fast 25Mhz processor, and was used initially in the high-end A540. A new desktop machine, the A5000, soon saw launch, with an ARM3 to power it. This was again a traditional desktop case, and offered 4MB of RAM and a 40/80MB hard drive. Complimenting this was another version of RISC OS - RISC OS 3.0.

By now, Acorn had established dominance in the educational market. At home, they maintained a small, if reasonable marketshare - at least in the Commonwealth. Acorn weren't resting on their laurels, however, and decided in 1991 to continue to evolve their product lines. The A3000, with its ageing ARM2, was axed, replaced by two new derivitives: the A3010 (aimed towards home use, with a video port to connect to a TV set and a joystick port) and the A3020 (intended for educational use). The ARM2 was swapped for the ARM250 (literally ARM 2 and a half), which boasted higher clock speeds, and integrated some of the custom circuitry of the A3000.

The A5000 was joined by the A4000, a lower-profile machine. RISC OS 3.1 was now standard on all machines, the last major revision of the OS until the release of the Risc PC in 1994. Acorn's Archimedes line served them well for the late-80's and early 90's, powerful machines with a wonderful OS. However, Acorn's terrible efforts at marketing prevented them from gaining much acceptance outside of education, which would prove to be their downfall.